Hmmm. Well, I would have a hard time writing a sequel with Frobisher in it because I don't really know his character -- certainly not well enough to sustain more than a brief appearance. So I'd have to go in a different direction, and instead write the Tenth Doctor in a series of vignettes where he tries to discover Zygons with his non-dinging Zygon detector. And of course, since he's terrible at it, he would fail miserably every time. He would go after more non-Zygon rabbits, and non-Zygon chickens, and there'd have to be a non-Zygon horse in there, and I'd probably throw in non-Zygon Jack Harkness just for fun. At the end he would give up, and then I'd have Zygon-Queen-Elizabeth show up to try to snog him to death with those venom sacs in her tongue, at which point someone would have to rescue him. It would probably be Frobisher, who was initially disguised as another Zygon.